L60 40mm Gun

Click on Picture to enlarge

Installation on an
AC-130

The 40mm Bofors Light Anti-Aircraft Gun, based on a
Swedish design and built in Canada, served in a number of different theatres
during the Second World War. A derivative design is mounted on the AC-130
gunship, with a shortened barrel and a bell-shaped blast deflector. A flexible
hood closes the gun mechanism from the outside and allows for cabin
pressurization. Starting in November 1969 gunships deployed in Vietnam had two
40mm guns in place of the two aft 20mm Vulcans which originally equiped these
aircraft. By 1972 the aft 40mm was replaced by a fixed-mounted 105mm Howitzer
on many aircraft.

The Bofors 40 mm gun is a
famous anti-aircraft auto-cannon designed by the Swedish firm of Bofors. It
was one of the most popular medium-weight anti-aircraft systems during World
War II, used by most of the western Allies as well as various other forces.
It is often referred to simply as the Bofors
gun

Development

The Swedish Navy purchased a number of 2
pounder
Pom-Poms from Vickers as anti-aircraft guns in 1922. Looking for a smaller
hand-swung weapon to complement the heavy Vickers, they asked Bofors A.B. to
develop a 20 mm weapon based on a similar mechanism (and generally similar to
the Vickers 1-pdr). Although this 20 mm design was not put into production,
the Navy quickly soured on the 2 pdr, and approached Bofors in 1929 about the
development of a much more capable replacement.

The Bofors 40 mm
Gun

Type

Auto-cannon

Place of origin

Sweden

Service history

Wars

WW2

Specifications

Crew

dependent on use

Shell

40 x 311R

Caliber

40 mm

Carriage

522 kg

Rate of fire

120 round/min

Muzzle velocity

2900 ft/s

Maximum range

7,160 m (AA)

Bofors was initially reluctant, the Swedish Navy being a
fairly small market, but the Navy eventually agreed to pay for the development
of a prototype. Bofors responded with a gun that was, to some extent, a
smaller version of a 57 mm (6-pounder) semi-automatic gun developed as an
anti-torpedo boat weapon in the late 1800s by Finspong before Bofors drove
them out of business. Their first prototype was in fact a re-barreled
Nordenfelt version of the Finspong gun, adding an auto-loader mechanism
similar to the Vickers "machine gun" system using a moving bolt.

Testing of the prototype in 1929 demonstrated the major
problem was feeding the weapon in order to maintain a reasonable rate of fire.
A bolt that was heavy enough to handle the stresses of firing the large round
was too heavy to move quickly enough to fire quickly. One interesting attempt
to solve this problem used zinc shell cases that burned up when fired. This
proved to leave heavy zinc deposits in the barrel, and had to be abandoned.
Instead they experimented with a newer mechanism that simply "threw" the
rounds into the breach from the rear without guiding them, the empty cases
simply falling out to the rear when the breach was opened. This proved to be
the solution they needed.

During this period Krupp purchased a one-third share of
Bofors. Krupp engineers started the process of updating the Bofors factories
to use modern equipment and metallurgy, but the 40 mm project was kept secret.
Nevertheless, many sources claim that the 40 mm design was in fact adapted
from a Krupp weapon. However the only German weapon of similar caliber (and
role) was the 3.7 cm FlaK 43, made by Rheinmetall; it appears historians
connected the development of the 40 mm and German 37 mm weapons without any
supporting evidence. It should be pointed out these two weapons are quite
different from each other and share few, if any, features.

By June 1930 testing with the prototype was complete, and
Bofors reported that full-scale development could begin. A prototype was
completed and fired in November 1931, and by the middle of the month it was
firing strings of two and three rounds. Changes to the feed mechanism were all
that remained, and by the end of the year it was operating at 130 rounds per
minute. Continued development was needed to turn it into a weapon suitable for
production, which was completed in October 1933. Since acceptance trials had
been passed the year before, this became known as the 40 mm akan M/32. Most
forces referred to it as the Bofors 40 mm L/60, although the barrel was
actually 56.1 calibers in length.

Click on Picture to enlarge

The gun fired a 2 lb (900 g) high explosive 40 x 311R
(rimmed) shell at 2,800 ft/s (850 m/s). The rate of fire was normally about
120 rounds per minute, which improved slightly when the barrels were closer to
the horizontal as gravity assisted the feeding. In practice firing rates were
closer to 80–100 rpm, as the rounds were fed into the breech from four round
clips which had to be replaced by hand. The maximum attainable ceiling was
23,600 ft (7,200 m), but the practical maximum was about 12,500 ft (3,800 m).

The gun was also notable due to its advanced sighting
system. The trainer and layer were both provided with reflector sights for
aiming, which a third crewmember standing behind them "adjusted" for lead
using a simple mechanical computer. Power for the sights was supplied from a
6V battery.

With the design now reaching completion, the Swedish Navy
once again decided it needed a smaller hand-swung weapon of 13 mm–25 mm, and
started testing various designs from foreign suppliers. With the 40 mm well
along in development, Bofors offered a 25 mm version in 1932, which was
eventually selected as the 25 mm akan M/32. The first version of the 40 mm the
Navy ordered featured a shorter barrel intended for use on submarines. The
barrel was shorter at 42 calibers long, with the effect of reducing the muzzle
velocity to about 700 m/s. When not in use, the gun was pointed directly up
and retracted into a watertight cylinder.

Interestingly the first order for the "real version" was
made by the Dutch Navy, who ordered five twin-gun mounts for the cruiser de
Ruyter in August 1934. These guns were stabilized using a unique system
known as the Hazemeyer mount, in which one set of layers aimed the gun,
while a second manually stabilized the platform the gun sat on. All five
mounts were operated by one fire control system.

Click on Picture to enlarge

A MK 12 quadruple mount
of Bofors guns fires from the USS Hornet

Bofors also started the development of a suitable towable
carriage which they displayed in April 1935 at a show in Belgium. The carriage
caused something of a stir, as the gun could be fired from the carriage with
no setup required, although with limited accuracy. If time was available the
gunners used the tow-bar and muzzle lock as levers, raising the wheels off the
ground and thereby lowering the gun onto supporting pads. Two additional legs
folded out to the sides, and the platform was then leveled with hand cranks.
The entire setup process could be completed in under a minute.

Orders for the land based versions were immediate,
starting with an order for eight weapons from Belgium in August 1935, and
followed by a flood of orders from other forces including Poland, Norway, and
Finland. It was only accepted into the Swedish Army the next year, known as
the 40 mm lvakan m/36, the lower-case m indicating an Army model as opposed to
the capital M for Navy.

The US Version

Click on Picture to enlarge

The Bofors 40mm gun
originated as a German Krupp design in 1918.

The WW 2 40mm was
manufactured by the Bofors Company in Sweden which, quite
literally hand built and fitted each weapon. Mass production
techniques were not used by Bofors. When the United States bought
the rights to manufacture the 40mm, we did employ mass production techniques which allowed us to produce the 40mm by the thousands.
The British produced the Mark 1 single barrel and the Mark II
double barrel. The United States produced single, double and quad
mounts of the 40mm and supplied them to all our allies.
The 40mm was the Navy's standard intermediate range
anti-aircraft weapon throughout WW 2, the Mk IV quad mount being
extremely effective. The 40mm was used post-war until fast-moving
jet aircraft became commonplace.

The US Navy's Bureau of Ordnance purchased a twin-mount
air-cooled example directly from Bofors, which arrived in New York on 28
August 1940. During that month another Dutch ship, the van Kinsbergen,
demonstrated the Hazemeyer mount to Navy observers. The gun was quickly chosen
as the Navy's standard anti-aircraft weapon, and the Navy secretly imported a
set of Imperial designs from England and started production illegally. A
formal contract with Bofors followed in June 1941. The resulting Mark 1 and
Mark 2 weapons were intended for the left and right side of a twin mount,
respectively.

The US Army had recently introduced a 37 mm gun of their
own design, but found it to be of limited performance. Six British guns were
imported for testing, along with the Kerrison Directors, and proved to be
superior in all areas. In order to supply both the Army and Navy with much
greater numbers of the guns, Chrysler was brought in to be a major producer.
Over the lifetime of the production, their engineers introduced numerous
additional changes to improve mass production, eventually reducing the overall
time needed to build a gun by half.

Click on Picture to enlarge

Continued Use

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Crewmen loading ammunition
in the Bofors of a Lockheed AC-130.

Although the L/60 was later replaced in production by the
L/70, the L/60 remained in front-line service well into the 1980s. In most
cases these were the airborne versions, as a suitable replacement in this role
did not come along until the introduction of truly effective MANPADS missiles
in the 1980s. L/60's are still used in the USAF's AC-130 gunships in the
air-to-ground role.

The L/60 saw active service with the Royal Navy in the
1982 Falklands War and continued to be used into the 1990s, when it was
replaced by modern 20- and 30-mm artillery. The Canadian Navy followed their
example, but removed the guns in the late 1980s when they were considered to
be outdated. In a somewhat embarrassing episode, the Navy was forced to scour
various military museums across Canada to re-equip their fleet during the Gulf
War, as replacements had never been purchased.

In 1993 a Bosnian L/60 engaged a Danish Leopard 1A5DK
tank from UNPROFOR leading to a few scratches in the tank's paint. Return fire
from the tank resulted in the complete destruction of the gun position.

Click on Picture to enlarge

Bofors 40 mm
Gun

In August 2006, the French navy uses L/60 on more than
twenty ships (patrols and auxiliaries).

Ships of the Norwegian and Icelandic Coast Guards use the
40mm Bofors gun to this day.

By the end of World War II, jet aircraft had so increased
the speed of attack that the Bofors simply could not get enough rounds into
the air to counter the aircraft before it had already flown out of range. In
order to counter these threats, the gun would have to have longer range and a
higher rate of fire, thereby increasing the number of rounds fired over the
period of an engagement. Bofors considered either updating the 40 mm, or
alternately making a much more powerful 57 mm design, and in the end did both.

The new 40 mm design used a larger 40 x 364R round firing
a slightly lighter 870g shell at a much higher 1,030 m/s muzzle velocity. The
rate of fire was increased to 240 rounds per minute, unusually high for such a
large round. Additionally the carriage was modified to be power-laid, the
power being supplied by a generator placed on the front of the carriage. The
first version was produced in 1947, accepted in 1948 as the 40 mm lvakan m/48,
and entered Swedish service in 1951. Additional changes over the years have
improved the firing rate first to 300 rpm, and later to 330.

Click on Picture to enlarge

Bofors 40mm L/70.

Foreign sales started, as they had in the past, with
Holland and Great Britain. In November 1953 it was accepted as the NATO
standard anti-aircraft gun, and was soon produced in the thousands. The L/70
was also used as the basis for a number of SPAAG's, notably the US Army's
infamous M247 Sergeant York, which proved to be unable to hit even hovering
drone targets.

Breda (now Oto Melara) of Italy uses the Bofors 40 mm
L/70 gun in its anti-aircraft weapon system for the Italian Army and Navy. A
newer development from Breda, the Fast Forty, has nearly doubled the rate of
fire to 450 rpm, normally equipped with a 43 round clip, or a 101 round clip
for naval use.

In the Swedish Army Combat Vehicle 90 there is a
cartridge fed, automatic version of the L/70 gun installed. It is also
interesting to note that the gun, in order to fit inside the vehicle, is
mounted upside down. New amour piercing and programmable ammunition have also
been developed.